The Effect of Short-Chain Branch Placement and Length on Oxidative Degradation in Bimodal High-Density Polyethylene

This study investigates the role of placement and length of the short-chain branch (SCB) on degradation, microstructural evolution, and ductility of bimodal high-density polyethylene (HDPE) during chemo-oxidative aging. Two resins, HDPE-B (with 1-butene comonomer) and HDPE-H (1-hexene comonomer), were analyzed using gel permeation chromatography (GPC-IR), Fourier transform infrared spectroscopy (FTIR), temperature rising elution fractionation (TREF), differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), extreme-small-angle X-ray scattering (ESAXS), and tensile testing. Although both samples exhibit similar initial bulk properties, differences in the SCB architecture led to distinct degradation processes. In HDPE-B, chain scissions in high molecular weight chains were followed by lamellar thickening, preserving ductility but limiting strain hardening. In contrast, HDPE-H exhibited increased crystallization and enhanced strain hardening due to chemicrystallization and scission near branches, resulting in reduced ductility. Microstructural models are proposed to explain how SCB-induced differences in the initial amorphous phase composition govern mechanical long-term behavior.